Foton optics at very high energy

In experimental nuclear physics quite often we need to localize very precisely the position of the nuclear reaction within the target, or, equivalently, the place of origin of the emitted reaction products. For that it is elementary to reduce the size of the target and the beam diameter as much as possible. The later can be done by focusing the beam.

At a conventional particle accelerator the charged particle beam can be focused on the target by quadrupole magnets. Submillimeter beam diamater can be easily achieved. For the focusing of photon beams one can use optical lenses: the focal distance of the lense is determined by the index of refraction of its material (n), its thickness at the middle (d) and the two radius of curvature (R1,R2). However, the index of refraction depends on the photon energy: n(E) = 1 − δ(E) + iβ(E), where iβ(E) stands for the absorption, and δ(E) drops very fast with energy! So at very high energy the focal distance (f) is divergent: for X-rays typically δ=10^-5 - 10⁻⁶, and in the γ energy region δ → 0. By increasing the number of lenses f can be reduced since f=n/(2Nd), where N is the number of lenses (see Figure). But for the energetic γ photons it does not help...

Array of lenses for focusing X-rays

Very recently a series of experiment performed by a group of physicists from the Ludwig Maximilians University and from Institut Laue-Langevin showed signatures on a deviation from the above mentioned tendency. The index of refraction can be different from 1 even for γphotons! A possible explanation is based on the so-called Delbrück scattering, which means an interaction between the photons and the electron-positron pairs those appear in the vicinity of nucleus with high atomic numbers.

We hope that in the near future already at ELI-NP the γ beam can be focused by using this effect with lenses having high atomic numbers (e.g. gold lenses).